Cisco IOS Wide-Area Networking Configuration Guide - Free Books

• User priority field• Canonical format indicator (CFI)• A 12-bit VLAN ID (VID)For L2TPv3, an Ethernet subinterface configured to support VLAN switching may be bound to anxconnect service so that all Ethernet traffic, tagged with a VID specified on the subinterface, is tunneledto another PE. The VLAN Ethernet frames are forwarded in their entirety. The receiving PE may rewritethe VID of the tunneled traffic to another value before forwarding the traffic onto an attachment circuit.To successfully rewrite VLANs, it may be necessary to disable the Spanning Tree Protocol (STP). Thiscan be done on a per-VLAN basis by using the no spanning-tree vlan command.NoteBecause of the way in which L2TPv3 handles 802.1q VLAN packets, the Ethernet interface must beconfigured in promiscuous mode to capture all traffic received on the Ethernet segment attached to therouter. All frames are tunneled through the L2TP pseudowire.HDLCL2TPv3 encapsulates an HDLC frame arriving at a PE in its entirety (including the Address, Control,and Protocol fields, but not the Flag fields and the frame check sequence) with an L2TP data header.PPPPEs that support L2TPv3 forward PPP traffic using a “transparent pass-through” model, in which thePEs play no role in the negotiation and maintenance of the PPP link. L2TPv3 encapsulates a PPP framearriving at a PE in its entirety (including the HDLC Address and Control fields) with an L2TP dataheader.ATML2TPv3 can connect two isolated ATM clouds over a packet-switched network (PSN) while maintainingan end-to-end ATM Service Level Agreement (SLA). The ATM Single Cell Relay features forward oneATM cell per packet. The ATM Cell Packing over L2TPv3 features allows multiple ATM frames to bepacked into a single L2TPv3 data packet. All packets are transparently forwarded over theL2TPv3 pseudowire.NoteVPI or VPI/VCI rewrite is not supported for any ATM transport mode. Both pairs of PE to CE peerrouters must be configured with matching VPI or VCI values except in OAM local emulation mode. Forexample, if PE1 and CE1 are connected by PVC 10/100, PE2 and CE2 should also be connected byPVC 10/100.Table 6 shows the releases that introduced support for the ATM cell relay features.Table 6Release Support for the ATM Cell Relay FeaturesTransport Type Single Cell Relay Packed Cell RelayVC mode 12.0(28)S, 12.2(25)S 12.0(29)S44